Ability to undo (and re-open) the merge of a pull-request

Sometimes pull-requests get merged by mistake, and although we can easily undo that in Git (git reset HEAD~1 + push --force or git revert + push), the pull-request is marked as merged forever... Which is inconsistent with that is in the source control.

I'd like to have a button on merged pull-requests that allow me to undo and reopen a pull-request (even if it doesn't do anything with the Git repository and I have to update the commits manually).